Waveguide systems or structures or parts thereof, containing polycyanate copolymers prepared from polyfunctional cyanates and fluorinated monocyanates

ABSTRACT

The present invention is directed to wave guide systems or structures or parts thereof, characterized in that they consist of or comprise a resin composed of at least one polycyanate copolymer, obtainable by copolymerization of at least one specific difunctional cyanate with at least one monocyanate of the formula N≡—C—O—R, wherein R is a straight or branched non-aromatic hydrocarbon radical or a non-aromatic hydrocarbon radical comprising a cyclic structure, the radical having the formula C(R′) 2 —CFR″ 2  wherein each R′ is, independently from the other, hydrogen or fluorine or an optionally substituted, preferably fluorinated alkyl or alkenyl group, and each of R″ may independently be defined as R′ or may have an arylic structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing of International ApplicationNo. PCT/EP00/06203, filed Jul. 3, 2000, which published in the Englishlanguage and claims priority of European Patent Application No.99112596.4, filed on Jul. 1, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to optical elements in the field ofwaveguide systems or waveguide structures, e.g. arrayed wave guidecomponents, prepared by copolymerization of specific polyfunctionalcyanates and fluorinated monocyanates, as well as to the use of saidcopolymers for the preparation of said structures.

Organic polymers are increasingly interesting materials in the opticalor microoptical field, in integrated optics or in microsystemtechniques. In these fields, they may be used in optical instruments andapparatuses or parts thereof as well as in special optics as lenses,prisms, for fixation of optical systems, as support material for opticallayers and as translucent coating materials for mirrors or lenses.Organic polymers may be used in optical fibres and for the preparationof waveguide structures. Their technical handling is relatively easy,and their density is lower in comparison to glass.

Specifically, if such plastics or organic polymers are to be used as awaveguide, a variety of requirements are to be met. The refractive indexof the material should be variable in a range as broad as possible andshould be adaptable to that of specific substrates. If used in theoptical communication engineering, low absorptions of the materials arerequired at 1.3 and 1.55 μm. The loss due to attenuation caused byvolume defects (non-homogenities, microbubbles, microfissures) should beminimized. Besides specific technological requirements, e.g. preparationof layers and structurability, specific provisions for the use oforganic polymers as waveguide structures in integrated optics are thethermal and thermo-mechanical stability, adapted extension coefficientsand long term stability of optical properties.

2. Description of the Related Art

Until now, polymethacrylates and polycarbonates have been mainly usedfor optical purposes. Both classes of polymers have an excellent lighttransmittance, but their thermal and thermo-mechanical stability is notsufficient due to their chemical structure. Thus, polymethacrylates andpolycarbonates cannot practically be used at temperatures exceeding 130°C. which is due to their relatively low glass transition temperatures.In addition, both types of polymers are linear, un-crosslinked polymers.This has the adverse effect that they are partly solubilized in casemultilayer-systems are prepared via the application of dissolvedcomponents, e.g. by spin-coating each layer. Consequently, the layerstructures as obtained are not sufficiently delimitated and neat which,however, is an essential for the preparation of waveguide structures.

There are other high performance polymers which have glass transitiontemperatures of more than 180° C. Examples are polyarylethersulfones,polyarylsulfones, polyaryletherketones, polyimides and polyetherimides,the processing of which, however, is more difficult than that ofpolymethacrylates and polycarbonates. Another disadvantage of thesesystems is the relatively high optical loss at wave lengths of 1.3 and1.55 μm, relevant in communication engineering.

Polyperfluorocyclobutanes (PFCB) are a relatively new class of highperformance polymers. Upon thermal curing they yield unsolublecross-linked polymers which are characterized by high thermal stability.Waveguide layers prepared from PFCB show very low optical losses of 0.2dB/cm at 1550 nm.

Also, polycyanurates have been used for the preparation of opticalcomponents. U.S. Pat. Nos. 5,208,892 and 5,165,959 describe thepreparation of polycyanate resins made of a single monomer (eitherfluorinated or non-fluorinated). German Offenlegungsschrift DE 44 35 992A1 describes optical elements prepared from polycyanurate resins. Theresins are made by polymerization of dicyanate or polycyanate compounds,optionally in mixture with di- or polyphenols or di- or polyglycidylcompounds. Like polyperfluorocyclobutanes, polycyanurates yieldunsoluble cross-linked polymers upon thermal curing, and these polymersare as well characterized by high thermal stability. They arespecifically useful due to their excellent adhesional force on a varietyof substrates, for example silicon, silica or a variety of organicpolymers. Refractive index and glass transition temperature of the curedcross-linked polymers may be varied in broad ranges, due to the easyavailability of a great number of di- and mono-functional cyanatemonomers which may be copolymerized with each other. Polycyanurates ofthe kind mentioned above are partly commercially available. Completelycured polycyanurates known in the art which consequently are stable forlong terms may have optical losses of about 0.2 dB/cm at 1.3 μm.However, the optical losses are not less than 0.5dB/cm at 1.55 μm whichis important in communication engineering technologies.

SUMMARY OF THE INVENTION

The present invention provides copolymers, obtainable bycopolymerization of at least one monocyanate, derived from a partly orfully fluorinated alcohol (“fluorinated monocyanate”), and at least onespecific difunctional organic cyanate. It has been found that suchcopolymers are specifically valuable in the preparation of opticalwaveguide systems or structures thereof having low optical losses at 1.3and at 1.55 μm.

Throughout the invention, “partly fluorinated” means that at least onefluorine atom is present in the molecule. “Fully fluorinated” means thathydrogen atoms are completely substituted by fluorine atoms. The wholemolecules, or single organic radicals or groups (e.g. methyl, methylene,alkyl, aryl groups), respectively, may be fully fluorinated.

DETAILED DESCRIPTION OF THE INVENTION

As fluorinated monocyanate, one, two, three or even more monocyanates offormula I may be used

N≡C—O—R  (I)

wherein R is C(R′)₂—CFR″₂, wherein each R′ is, independently from theother, hydrogen or fluorine or an optionally substituted, preferablyfluorinated alkyl or alkenyl group having preferably 1 to 13, morepreferably 3 to 11 carbon atoms. Each of R″ may independently be definedas R′. Further, R″ may have an arylic structure. Preferably, R is astraight, branched, or cyclic non-aromatic hydrocarbon radical or annon-aromatic hydrocarbon radical comprising a cyclic structure.Preferably, the non-aromatic hydrocarbon radical has 1 to 15, morepreferably 3 to 12 carbon atoms. It is to note that each of the carbonatoms of R may carry 1, 2 or, if it is a terminal carbon atom, 3fluorine atoms. Fully fluorinated carbon atoms (—CF₃, —CF₂—) arepreferred. Further, it is preferred that one or both of R′ are hydrogenand/or one of R″ is fluorine or a partly or fully fluorinated alkyl andthe other is a partly or, more preferable, fully fluorinated alkyl whichmay be straight, branched or cyclic. Specific examples for the cyanatesof formula (I) are —CH₂—CF₂—CF₃, —CH₂—CF₂—CF₂—CF₃, —CH₂—C(CF₃)₂F,—CH₂—CF₂—CF₂—CF₃.

For the preparation of the said copolymer, one, two, three or even moredifunctional organic cyanates may be used. The expression “difunctional”means that two NCO groups are present in the molecule. The NCO groupsare bound to organic radicals via the oxygen atom. The difunctionalcyanate may be, but is not necessarily, partly or fully fluorinated. Theorganic structure of the difunctional cyanate or cyanates is selectedunder difunctional cyanates of formula II:

wherein R¹ to R⁴ and R⁵ to R⁸ are independently from each otherhydrogen, optionally substituted C₁-C₁₀ alkyl, C₃-C₈-cycloalkyl,C₁-C₁₀-alkoxy, halogen, phenyl or phenoxy, the alkyl or aryl groupsbeing unfluorinated, partly fluorinated or fully fluorinated, Z is achemical bond, SO₂, CF₂ CH₂, CHF, CH(CH₃), isopropylene,hexafluoroisopropylene, n- or iso-C₁-C₁₀ alkylene which may be partly orfully fluorinated, O, NR⁹ with R⁹ being hydrogen or C₁-C₁₀ alkyl, N═N,CH═CH, C(O)O, CH═N, CH═N—N═CH, alkyloxyalkylene having 1 to 8 carbonatoms which is optionally partly or fully fluorinated, S, or Si(CH₃)₂.Examples are

2,2′-bis(4-cyanato-phenyl)propane,

2,2′-bis (4-cyanato-phenyl)hexafluoropropane,

biphenylene-4,4′dicyanate,

2,3,5,6,2′,3′,5′,6′octafluorobiphenylene-4,4′-dicyanate.

In one embodiment of the invention, dicyanates according to formula III:

N≡C—O—R¹⁰—O—C≡N  (III)

wherein R¹⁰ is an organic non-aromatic hydrocarbon group carrying atleast 1 fluorine atom are copolymerized into the polycyanate copolymeruseful in the present invention. In formula (III), R¹⁰ is preferably analkylene group, more preferably having 3 to 12 carbon atoms. Each of thecarbon atoms may carry 0, 1 or 2 or, in the case of a terminal group, 3fluorine atoms. The carbon chain may be straight or branched or may becyclic or may contain a cyclic part. Further, it may contain one or moreC═C double bonds. In one embodiment, R¹⁰ is fully fluorinated. Examplesare —CH₂—CF₂—CF₂—CH₂— or —CH₂—CF₂—CF₂—CF₂—CF₂—CH₂.

In another embodiment of the present invention, at least one additionalmonocyanate having formula IV:

wherein R¹ to R⁵ are as previously defined for formula II, iscopolymerized in addition to the starting cyanates as defined above(with or without (a) cyanate(s) of formula III) in order to obtain thepolycyanate copolymer. Examples for compounds of formula IV arephenylcyanate and perfluorophenylcyanate.

Specifically, the refractive index and the glass transition temperaturemay be influenced by this additive as desired.

The polycyanate copolymers according to the invention may be obtained bymixing at least one of the monocyanates of formula I, optionally inaddition to at least one of formula IV, and at least one difunctionalorganic cyanate of formula II, optionally in addition to at least one offormula III. The ratio of monocyanates to dicyanates may be freelychosen, provided that at least 1% by mol, preferably at least to 5% bymol, more preferably at least 10% by mol of monocyanate of structure Iis present per mol of monomers to be polymerized. Preferably, themonofunctional cyanates of formulas I and IV are present in a molaramount of not more than 75% related to the total amount of moles ofmonomers present in the mixture to be copolymerized.

The starting monocyanate and dicyanate compounds as described above arepreferably warmed up after mixing. The temperature may be chosen asrequired; a range of about 120° C. to 170° C. is preferred. Preferably,the reaction is performed in the absence of oxygen, e.g. in a sealed andpreferably (under an inert gas atmosphere). The mixture is allowed toreact until a liquid or viscous prepolymer (resin) is obtained. Thisprepolymer or resin is soluble in useful solvents, preferably insolvents having high polarity, e.g. ethylethoxyacetate or chlorobenzene.In general, the prepolymer is processed in a respective solution, e.g.by spin-coating of a solution containing 25 to 65% by weight of theprepolymer, more preferably about 50% by weight of the prepolymer. Theprepolymer solution may be applied to a suitable substrate, consistingof e.g. silicon, quartz or an organic polymer. After being brought intothe desired shape (e.g. a layer of desired thickness) it is cured (e.g.at temperatures in the range of 200° to 260° C.) in order to provide thedesired network between the cyanate groups.

If an optical wave guide system comprising a variety of different layersof the present polycyanate copolymers shall be prepared, each differentlayer is applied and is cured, e.g. thermically cured, before the nextlayer is applied.

It shall be clear that the term “resin” is independent of the conditionof the polymer, e.g. whether it is in a prepolymerized condition or ispartly or completely cured.

The polycyanate copolymers according to the present invention have aglass transition temperature in the range of 100° to 300° C., and theirrefractive index at 1.55 μm may be controlled in the desired range,specifically of from 1.35 to 1.60. Specifically, the more fluorinatedmonomers are used, or the more fluorine parts per weight are present inthe mixture, related to the weight of the mixture to be polymerized, thelower is the refractive index of the polycyanate copolymer obtained.

On the other hand, use of brominated derivatives of the cyanate monomersas defined above will raise the refractive index of the copolymerobtained. Thus, monocyanate compounds of e.g. formula IV wherein atleast one of R¹, R², R³, R⁴ o4 R⁵ is substituted by bromine, may beadvantageously added to the mixture. In general, the more bromine isincluded in the polymer, the higher is the refractive index obtained.Accordingly, any of the cyanates of formulas I to IV as defined above(with the proviso that those of formula I may be free of fluorine)carrying one or more bromine atoms may be selected. However, brominatedmonocyanates are preferably used, either alone or in mixture withbrominated polycyanates.

The polycyanate copolymers according to the present invention are usedfor the preparation of optical wave guide systems or parts thereof. Forexample, they may be used for the preparation of waveguides andwaveguide structures. For such structures, use of at least two differentpolycyanate copolymers is preferred, wherein a polycyanate copolymerhaving a lower refractive index may be used for buffer and/or claddingwhile a polycyanate copolymer differing from the first one and having agreater refractive index may be used as the optical waveguide. At leastone of these polycyanate copolymers should have been obtained accordingto the present invention. The selection will be easily made by a skilledperson who is able to control the refractive index via the teachingsgiven in this application. The layers show excellent adhesion to eachother and to the substrate. Waveguide structures as described above maybe prepared by known methods, e.g. RIE (Reactive Ion Etching).

The invention is now further illustrated by way of examples.

EXAMPLE 1

12.9 g of a substituted dicyanate of Bisphenol A (compound II whereinR¹-R⁴ is H, R⁵-R⁸ is H, Z is hexafluoroisopropyl) and 3.7 g of a partlyfluorinated monocyanate (compound I wherein R is CH₂—CF₂—CF₂—CF₃) areheated to 160° C. in a sealed vessel for a time of about four hours. Thereaction is terminated before gelling starts, and a clear, pale yellowprepolymer is obtained which is viscous at 160° C. and is solid at roomtemperature. The prepolymer is brought into solution by mixing it with50% by weight of ethylethoxyacetate (EEA). Spin-coating of this solutiononto a substrate made of silicon wafer yields a layer which may be curedat 240° C. for one hour in a drying oven. The product has a refractiveindex of 1.4776 at 1.55 μm.

EXAMPLE 2

12.9 g of a substituted dicyanate of Bisphenol A (compound II whereinR¹-R⁴ is H, R⁵-R⁸ is H, Z is hexafluoroisopropyl), 3.7 g of a partlyfluorinated monocyanate (compound I wherein R is CH₂—CF₂—CF₂—CF₃), and1.3 g of a monocyanate (compound IV wherein R¹, R², R⁴, R⁵ are hydrogenand R³ is bromine) are heated to 160° C. in a sealed vessel for a timeof about four hours. The reaction is terminated before gelling starts,and a clear, pale yellow prepolymer is obtained which is viscous at 160°C. and is solid at room temperature. The prepolymer is brought intosolution by mixing it with 50% by weight of EEA. Spin-coating of thissolution onto a substrate made of silicon wafer yields a layer which maybe cured at 240° C. for one hour in a drying oven. The product has arefractive index of 1.4870 at 1.55 μm.

EXAMPLE 3

9.7 g of dicyanate of Bisphenol A (compound II wherein R¹-R⁴ is H, R⁵-R⁸is H, Z is isopropyl) and 2.5 g of a fully fluorinated monocyanate(compound I wherein R is CH(CF₃)₂) are heated to 140° C. in a sealedvessel for a time of about four hours. The reaction is terminated beforegelling starts, and a clear, pale yellow prepolymer is obtained which isviscous at 140° C. and is solid at room temperature. The prepolymer isbrought into solution by mixing it with 50 % by weight of EEA.Spin-coating of this solution onto a substrate made of silicon waferyields a layer which may be cured at 240° C. for one hour in a dryingoven. The product has a refractive index of 1.5596 at 1.55 μm.

EXAMPLE 4

9.7 g of a substituted dicyanate of Disphenol A (compound II whereinR¹-R⁴ is H, R⁵-R⁸ is H, Z is hexafluoroisopropyl), 10.3 g of a partlyfluorinated dicyanate (compound III wherein R¹⁰ isCH₂—CF₂—CF₂—CF₂—CF₂—CH₂) and 1.1 g of a partly fluorinated monocyanate(compound I wherein R is CH₂—CF₂—CF₂—CF₃) are heated to 140° C. in asealed vessel for a time of about four hours. The reaction is terminatedbefore gelling starts, and a clear, pale yellow prepolymer is obtainedwhich is viscous at 140° C. and is solid at room temperature. Theprepolymer is brought into solution by mixing it with 50 % by weight ofEEA. Spin-coating of this solution onto a substrate made of siliconwafer yields a layer which is cured at 240° C. for one hour in a dryingoven. The product has a refractive index of 1.3689 at 1.55 μm.

EXAMPLE 5

A 50 weight-% solution of the prepolymer of example 1 in EEA isspin-coated onto a silicon wafer, yielding a layer of about 8 μmthickness. Curing is performed at 240° C. in a drying oven for one hour.Onto this layer, a 50 weight-% solution of the prepolymer of example 2in EEA is spin-coated, again yielding a layer of about 8 μm thickness.Also, this layer is cured at 240° C. in the drying oven for about 1hour. According to known methods, an aluminum layer of about 100 nm issputtered onto the said second prepolymer layer followed by itsstructurization by way of photolithography and chemical etching.Subsequently, the waveguides are structured by aid of oxygen RIEtechniques (typical rate 100 nm/min using pure oxygen), and the etchingmask is removed by treatment in a chemical etching bath. Then, the uppercladding layer is applied by spin-coating a prepolymer solution ofexample 1 followed by curing at 240° C. for 1 hour. Using near fieldtechnique a difference of 0.0094 of the refractive index between thewaveguide and its surrounding is measured. Cut-back measurements oflight intensities of waveguides of different lenght yielded a loss of0.35 dB/cm at 1.55 μm.

What is claimed is:
 1. An optical waveguide system or a structure orpart thereof, comprising a resin composed of at least one polycyanatecopolymer, obtainable by copolymerization of at least one difunctionalcyanate of formula II:

wherein R¹ to R⁴ and R⁵ to R⁸ are independently from each otherhydrogen, optionally substituted C₁-C₁₀ alkyl, C₃-C₈-cycloalkyl,C₁-C₁₀-alkoxy, halogen, phenyl or phenoxy, the alkyl or aryl groupsbeing unfluorinated, partly fluorinated or fully fluorinated, Z is achemical bond, SO₂, CF₂CH₂, CHF, CH(CH₃), isopropylene,hexafluoroisopropylene, n- or iso-C₁-C₁₀ alkylene, O, NR⁹, N═N, CH═CH,C(O)O, CH═N, CH═N—N═CH, alkyl oxyalkylene having 1 to 8 carbon atoms, S,Si(CH₃)₂, and R⁹ is hydrogen or C₁-C₁₀ alkyl with at least onemonocyanate of the following formula I: N≡C—O—R  (I) wherein R is astraight or branched non-aromatic hydrocarbon radical or a non-aromatichydrocarbon radical comprising a cyclic structure, the radical havingthe formula C(R′)₂-CFR″₂ wherein each R′ is, independently from theother, hydrogen or fluorine or an optionally substituted alkyl oralkenyl group, and each of R″ may independently be defined as R′ or mayhave an arylic structure.
 2. An optical waveguide system or a structureor part thereof according to claim 1 wherein the substituted alkyl oralkenyl group of R′ is fluorinated.
 3. An optical waveguide system or astructure or part thereof according to claim 1, wherein the polycyanatecopolymer is obtainable by copolymerization of at least one difunctionalcyanate of formula II, at least one monofunctional cyanate of formula Iand at least one dicyanate having formula III: N≡C—O—R¹⁰—O—C≡N  (III)wherein R¹⁰ is a non-aromatic hydrocarbon group carrying at least onefluorine atom.
 4. An optical waveguide system or a structure or partthereof according to claim 3, wherein R¹⁰ of formula III is a partly orfully fluorinated alkylene group having 1 to 15 carbon atoms.
 5. Anoptical waveguide system or a structure or part thereof according toclaim 4, wherein the partly or fully fluorinated alkylene group has 3 to12 carbon atoms.
 6. An optical waveguide system or a structure or partthereof according to claim 3, wherein the polycyanate copolymer isobtainable by copolymerization of at least one difunctional cyanate offormula II, at least one monofunctional cyanate of formula I, optionallyat least one dicyanate having formula III, and a monocyanate of formulaIV

wherein R¹ to R⁵ are defined as in formula II.
 7. An optical waveguidesystem or a structure or part thereof according to claim 3, wherein thepolycyanate copolymer is obtainable by copolymerization of at least onedifunctional cyanate of formula II, at least one monofunctional cyanateof formula I, and at least one brominated monocyanate of formulas I toIII, as defined above with the proviso that the brominated monocyanatesof formula I may be free of fluorine.
 8. An optical waveguide system ora structure or part thereof according to claim 7, wherein the at leastone brominated monocyanate is of formula I as defined above with theproviso that the monocyanates of formula I may be free of fluorine. 9.An optical waveguide system or a structure or part thereof according toclaim 1, wherein the monocyanate of formula I is used in an amount of atleast 10 mol % per mol of the polycyanate copolymer.
 10. An opticalwaveguide system or a structure or part thereof according to claim 1,wherein the monocyanate of formula I is used in an amount of at least 20mol % per mol of the polycyanate copolymer.
 11. An optical waveguide ora structure or part thereof according to claim 1, wherein thepolycyanate copolymer has a glass transition temperature of from 100° C.to 300° C. or a refractive index of about 1.35 to about 1.60 at 1.55 μm.12. An optical waveguide system or a structure or part thereof accordingto any one of claims 1 to 11, wherein the system, structure or partthereof is an optical fibre, a waveguide, a buffer layer, a cladding ora support for any of said structures.
 13. An optical waveguide systemcomprising a waveguide consisting of a resin as defined in any one ofclaims 1 to 11, and a buffer or cladding consisting of a resin asdefined in any one of claims 1 to 11, but different from the resin ofthe waveguide, wherein the resin of the waveguide has a greaterrefractive index than that of the buffer or cladding.
 14. An opticalwaveguide or a structure or part thereof according to claim 1, whereinthe polycyanate copolymer has a glass transition temperature of from100° C. to 300° C. and a refractive index of about 1.35 to about 1.60 at1.55 μm.